1. FIELD OF THE INVENTION
This invention relates to so-called hot isostatic pressing process (hereinafter referred to as HIP process) in which powder molded products of ceramic or metal are sintered or densified under a gas atmosphere of high temperature and high pressure.
2. DESCRIPTION OF THE PRIOR ART
The HIP process has attracted increasing attention in recent years in various fields of art as an excellent technique for producing sintered products of high density from powdered ceramics, powdered metals or a mixture thereof, eliminating residual pores in cemented carbides under collapse or forming diffusion bonds between metal materials by the isotropic compression of bodies to be processed at high temperature using an inert gas as a pressurizing medium. Since the HIP process is applied under a gas atmosphere at high temperature and high pressure, it has to be carried out using an expensive HIP apparatus equipped with special structures and over a long cycle time period for operation including the steps of temperature elevation, pressure increase, temperature lowering and pressure decrease. Thus, an important technical problem at the present time is to shorten the cycle time period in order to improve the productivity of the HIP process. For solving the above problem, an attempt has been proposed for the effective utilization of the HIP apparatus in view of its operation time by carrying out the time-consuming heating step of temperature elevation in a preheating furnace and, subsequently, carrying out the pressure increase step solely or in combination with only a slight temperature elevation in the HIP apparatus. Although this proposed method can be applied advantageously for those materials, for example, powdered metals encapsulated in steel capsules or cast materials having casting defects that require no particular consideration of the preheating atmosphere by using a preheating furnace of a relatively simple structure, such a simple preheating furnace can not be used for the process requiring vacuum or like other particular atmospheres, for instance, in the preliminary sintering of molded products of ceramic or metal powder, followed by a further densifying step in the HIP process. The method of applying the preliminary sintering and the subsequent HIP process to the powder molded products is an important process for producing cemented carbide tools, ceramic tools, ferrite materials and the likes. However, since the preliminary sintering is usually carried out in a preheating furnace of a complicated structure exclusively designed therefor and the sintered products are charged into the HIP apparatus after being cooled once near the room temperature, it results in a great loss in heat energy. In addition, preheating of the sintered products in the HIP apparatus brings about the increase in the time occupying the HIP apparatus, that is, the cycle time for the HIP process.
In order to avoid the heat energy loss in the HIP process, Japanese Patent Laid-Open No. 25061/1972 proposes a furnace for vacuum sintering and pressurization, which applies both the vacuum sintering and the HIP process in one apparatus. However, the powder molded products have to first be sintered under vacuum into presintering products in the HIP apparatus according to this proposed technique. As is well known there is a great time difference for elevating the temperature to a required level for the preliminary sintering between a pressurized state and an atmospheric or reduced pressure state and that a much longer time period is necessary under the atmospheric or reduced pressure state than under the pressurized state. For instance, argon gas at high pressure of 1000 kg/cm.sup.2 has a viscosity only 1.1-3 times as great as that of argon gas under atmospheric pressure although its density is several hundreds times as high as the latter. Therefore, violent convection occurs to remarkably increase the rate of the heat transfer through convection, whereby the heat transfer rate from the furnace atmosphere to the material to be sintered is significantly increased. Accordingly, if the preliminary sintering that has to be carried out under vacuum or inert gas, at most, at the atmospheric pressure is applied in the HIP apparatus, it would increase the time for temperature elevation due to the reduction in the heat transfer rate and further require additional time for exhaustion and replacement of furnace gas, which remarkably worsens the working efficiency of the relatively expensive HIP apparatus and leads to an increase in the cost of the products. Since the size of the HIP apparatus main body has been scaled-up and thus the HIP cycle time has been further increased due to the recent increasing trend in the products, various counter measures for decreasing the cycle time are taken for the improvement in the HIP apparatus, such as the use of a compressor with improved efficiency and increased capacity, improvement in a heating device and the like. However, the improvement in the apparatus per se has a certain limit due to the increased cost.